1. Field of the Invention
This invention relates to a photoelectric switch wherein pulse light emitted from light emitting means is detected by light detecting means for the purpose of detecting presence or absence of an object or the like.
2. Description of the Prior Art
A photoelectric switch of the above-described type comprises a light emitting section for emitting pulse light and a light detecting section detecting the pulse light emitted by the light emitting section to thereby determine absence or presence of an object. The light detecting section comprises a light receiving element receiving the emitted pulse light, an amplifying circuit, an integrating circuit and a comparing circuit. Upon detection of the pulse light, the light receiving element converts the light energy to corresponding electrical energy, thereby generating an electric signal. The electric signal is amplified by the amplifying circuit and the amplified signal is integrated by the integrating circuit. The comparing circuit compares the integral value with a reference value and generates a detection signal indicative of detection of the pulse light when the integral value is equal to or above the reference value. In the photoelectric switch of a through-beam type, for example, no detection signal is obtained when an object interrupting the axis of an optical detecting path is present, so that the presence of the object can be detected. Furthermore, an additional comparing circuit is provided for comparing the integral value from the integrating circuit with a higher stable operation level. The additional comparing circuit generates a stable operation signal when the integral value is equal to or above the stable operation level. Thus, the state that a sufficient intensity of light is impinging upon the light detecting section can be detected.
Since the electric signal converted by the light receiving element is weak in the above-described arrangement, it is easily affected adversely by electrical noise until it is amplified by the amplifying circuit. In view of this problem, the intensity of light emitted from the light emitting section is increased or the distance between the light receiving element and the amplifying circuit is shortened. Recently, the prior art has provided for a light detecting unit formed into an integral one-chip composition comprising the light receiving element, amplifying circuit, integrating circuit and comparing circuit. In this arrangement, the distance between the light receiving element and the amplifying circuit is so short that the electric signal converted by the light receiving element is not affected adversely by the above-described electrical noise. Furthermore, the comparing circuit can be composed to generate a high-level detection signal, for example when the integral value has exceeded a predetermined level. Consequently, the detection output can be used without any processing. Additionally, the light detecting unit can be simplified and rendered small.
In the above-described one-chip light detecting unit, however, the incorporated comparing circuit is configured to necessarily compare the integral value from the integrating circuit with only one fixed reference value, thereby generating a detection signal. Accordingly, the integral value cannot be compared with another higher level and the stable operation signal as described above cannot be obtained. Furthermore, even in the above-described light detecting unit, the detection signal is generated even when a noise or an extraneous light having a relatively short period impinges upon the light receiving element and the intensity of light detected by the light receiving element has exceeded the predetermined level. When the detection signal thus obtained is used for other purposes without any processing, a false detection would occur.